The present invention relates to rotors for caliper disc brakes and the like, and in particular to an electric discharge machine for producing brake components and a method for making same.
Rotors are generally well known in the art, and are used extensively in vehicle braking systems, power transmission devices, clutches, and other similar machinery and mechanisms. Vehicle caliper disc braking systems slow the vehicle by inhibiting the rotation of the vehicle wheels. Rotors used in typical vehicle braking systems include a central hat section for attaching the rotor to a vehicle wheel and drive member for rotation therewith, and an outer friction section having opposite friction surfaces.
A caliper assembly is secured to a non-rotating component of the vehicle, such as the vehicle frame. The caliper assembly includes a pair of brake pads disposed adjacent the rotor friction surfaces, and a moveable piston operatively connected to one or more of the brake pads. When the driver brakes the vehicle, hydraulic or pneumatic forces move the piston which clamps the pads against the friction surfaces of the rotating rotor. As the brake pads press against the moving rotor friction surfaces, frictional forces are created which oppose the rotation of the wheels and slow the vehicle. The friction converts the vehicle's kinetic energy into large quantities of heat, much of which is absorbed by the friction surfaces and conducted to the rest of the rotor and to other components to which the rotor is connected.
Brake rotors are typically cast from a ferrous material, such as cast or grey iron, and are then machined in multiple operations to shape the rotor, to form the inner hub portion and friction surfaces. However, ferrous material rotors are relatively heavy and they corrode during normal use. Brake rotors are also cast from aluminum based metal matrix composite (MMC) containing silicon carbide particulate reinforcement. Aluminum MMC rotors have sufficient mechanical and thermal properties at a significantly reduced weight compared to ferrous metal rotors. Typically, the rotor is cast from aluminum MMC and then machined in a conventional manner to form the finished rotor.
During conventional machining, a tool is pressed against the part to remove a portion of the surface of the part. However, conventional machining offers a disadvantage in that the physical contact between the tool and the part partially deforms the part during machining producing imprecision in the finished parts. For example, it is desirable to produce rotors having flat friction sections. Variations in the surface of the friction section produces undesirable brake noise, pedal pulsations, and non-uniform wear.
Additionally, the particulate reinforcement in aluminum MMC parts is very hard which makes the aluminum MMC castings difficult to machine. Special cutting tools made from expensive materials such as polycrystalline diamond are needed to machine aluminum MMC, yet the tools still tend to wear quickly which increases production costs. It is desirable to produce brake components, such as metal rotors, made from materials such as cast iron or aluminum MMC using an apparatus and technique which will reduce production costs while improving the tolerances of the parts.
Electric discharge machining (EDM) is a known method of machining metal parts using electric sparks. The electric sparks are directed against the surface to be machined. A high temperature is reached where the spark contacts the metal surface. The high temperature vaporizes the metal at that location. A series of sparks are directed at the surface to burn away a portion of the metal resulting in a finish machined part.
EDM offers advantages over conventional machining in that the EDM apparatus does not physically contact the part thereby improving the tolerances of the finished part. However, known EDM apparatus and machining techniques are slow, typically producing only about 5,000 sparks per second. The number of sparks produced per unit time in part determines how quickly the part can be machined. Conventional EDM apparatus are too slow to be cost effective for use in mass production. It is desirable to provide an apparatus and a method for machining metal brake components such as cast iron or aluminum MMC rotors using electrically discharged sparks which is quick and cost effective.